THREE-DIMENSIONAL ARTHROPODS FROM THE MICROLAMINATED SEDIMENTS OF THE BARSTOW FORMATION, MOJAVE DESERT, CALIFORNIA

Three-dimensional, mineralized (usually silicified) arthropods, crustaceans, arachnids, and plants are well known from the middle Miocene Barstow Formation of southern California. These microfossils are usually recovered from carbonate concretions by acid dissolution of the carbonate. Previous authors have described three-dimensional microfossils from concretion sites in the Calico Mountains (14 km east of Barstow) and in the Mud Hills (16 km north of Barstow). Previous reports have described a concretion derived fossil assemblage rich in brine shrimp, chironomid midge pupa, and dytiscid beetle larvae. Brine shrimp or midge fly fecal pellets are commonly found in dense layers within the concretions. Although these fossils are most common, a great variety of other three-dimensional fossils are found in lesser quantities including dragonfly nymphs, ostracods and plants. According to previous authors the fossils are found within the concretions and are not found in the surrounding sediments. In this paper, three-dimensional fossils are reported from the microlaminated sediments that surround the carbonate concretions. This newly recognized fauna includes dytiscid beetle larvae, a dragonfly nymph, ostracods, and abundant fecal pellets arranged along bedding planes of the microlaminated sediments. Carbonized plant fragments are also recognized from the microlaminated sediments. The fauna and flora recovered from the microlaminated sediments appears to be similar in composition to the fauna and flora recovered from the carbonate concretions. The fossils are not abundant and are composed of fiberous or sparry calcite which infills impressions (of arthropods) in the microlaminated sediments. The new fossils occur in the Calico Mountains in the Middle Member of the Barstow Formation. Marker beds (including the Red Tuff and Oreodont Tuff) allow correlation of the Calico Mountains fossils with the type section of the Barstow Formation in the Mud Hills.